registration.rs

// Copyright (C) Parity Technologies (UK) Ltd.
// This file is part of Parity Bridges Common.

// Parity Bridges Common is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Parity Bridges Common is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.

//! Bridge relayers registration and slashing scheme.
//!
//! There is an option to add a refund-relayer signed extension that will compensate
//! relayer costs of the message delivery and confirmation transactions (as well as
//! required finality proofs). This extension boosts priority of message delivery
//! transactions, based on the number of bundled messages. So transaction with more
//! messages has larger priority than the transaction with less messages.
//! See `bridge_runtime_common::priority_calculator` for details;
//!
//! This encourages relayers to include more messages to their delivery transactions.
//! At the same time, we are not verifying storage proofs before boosting
//! priority. Instead, we simply trust relayer, when it says that transaction delivers
//! `N` messages.
//!
//! This allows relayers to submit transactions which declare large number of bundled
//! transactions to receive priority boost for free, potentially pushing actual delivery
//! transactions from the block (or even transaction queue). Such transactions are
//! not free, but their cost is relatively small.
//!
//! To alleviate that, we only boost transactions of relayers that have some stake
//! that guarantees that their transactions are valid. Such relayers get priority
//! for free, but they risk to lose their stake.

use crate::RewardsAccountParams;

use codec::{Decode, Encode, MaxEncodedLen};
use scale_info::TypeInfo;
use sp_runtime::{
	traits::{Get, Zero},
	DispatchError, DispatchResult,
};

/// Relayer registration.
#[derive(Copy, Clone, Debug, Decode, Encode, Eq, PartialEq, TypeInfo, MaxEncodedLen)]
pub struct Registration<BlockNumber, Balance> {
	/// The last block number, where this registration is considered active.
	///
	/// Relayer has an option to renew his registration (this may be done before it
	/// is spoiled as well). Starting from block `valid_till + 1`, relayer may `deregister`
	/// himself and get his stake back.
	///
	/// Please keep in mind that priority boost stops working some blocks before the
	/// registration ends (see [`StakeAndSlash::RequiredRegistrationLease`]).
	pub valid_till: BlockNumber,
	/// Active relayer stake, which is mapped to the relayer reserved balance.
	///
	/// If `stake` is less than the [`StakeAndSlash::RequiredStake`], the registration
	/// is considered inactive even if `valid_till + 1` is not yet reached.
	pub stake: Balance,
}

/// Relayer stake-and-slash mechanism.
pub trait StakeAndSlash<AccountId, BlockNumber, Balance> {
	/// The stake that the relayer must have to have its transactions boosted.
	type RequiredStake: Get<Balance>;
	/// Required **remaining** registration lease to be able to get transaction priority boost.
	///
	/// If the difference between registration's `valid_till` and the current block number
	/// is less than the `RequiredRegistrationLease`, it becomes inactive and relayer transaction
	/// won't get priority boost. This period exists, because priority is calculated when
	/// transaction is placed to the queue (and it is reevaluated periodically) and then some time
	/// may pass before transaction will be included into the block.
	type RequiredRegistrationLease: Get<BlockNumber>;

	/// Reserve the given amount at relayer account.
	fn reserve(relayer: &AccountId, amount: Balance) -> DispatchResult;
	/// `Unreserve` the given amount from relayer account.
	///
	/// Returns amount that we have failed to `unreserve`.
	fn unreserve(relayer: &AccountId, amount: Balance) -> Balance;
	/// Slash up to `amount` from reserved balance of account `relayer` and send funds to given
	/// `beneficiary`.
	///
	/// Returns `Ok(_)` with non-zero balance if we have failed to repatriate some portion of stake.
	fn repatriate_reserved(
		relayer: &AccountId,
		beneficiary: RewardsAccountParams,
		amount: Balance,
	) -> Result<Balance, DispatchError>;
}

impl<AccountId, BlockNumber, Balance> StakeAndSlash<AccountId, BlockNumber, Balance> for ()
where
	Balance: Default + Zero,
	BlockNumber: Default,
{
	type RequiredStake = ();
	type RequiredRegistrationLease = ();

	fn reserve(_relayer: &AccountId, _amount: Balance) -> DispatchResult {
		Ok(())
	}

	fn unreserve(_relayer: &AccountId, _amount: Balance) -> Balance {
		Zero::zero()
	}

	fn repatriate_reserved(
		_relayer: &AccountId,
		_beneficiary: RewardsAccountParams,
		_amount: Balance,
	) -> Result<Balance, DispatchError> {
		Ok(Zero::zero())
	}
}